Rare transitions between different turbulent states have been reported for many different flows [1]}, [2]}, [3]}, [4]}. Similar phenomena are also important in many geophysical flows [5]}, [6]}. Here we have thus found another example of such rare transition events in turbulent flows, which were not identified for OB cases [7]}. Similar to prior studies [3]}, [4]}, such phenomenon can be viewed as flow mode competition between different states: for not very large \(\theta _m\) , e.g., \(\theta _m=0.93\) , the effective aspect ratio of the convection region is close to \(\Gamma =2\) , therefore, it only supports a two-roll state. At large \(\theta _m\)  close to \(\theta _m=1\) , the effective aspect ratio becomes much larger than 1, and therefore it can support more convection rolls [7]}. In between, the different convection roll states may have comparable strength, and the competition of these states leads to the rare transitions between them.
